Rights activists visiting abandoned government prisons in the first Syrian city to come under rebel control have found torture devices and other evidence that detainees were abused there, Human Rights Watch said in a report Friday.

Raqqa, in eastern Syria, was overrun in late February by rebels fighting to topple President Bashar Assad. The rebels facilitated the New York-based group's access to facilities that had belonged to a government security agency and military intelligence in late April.

The HRW said its researchers found physical evidence that Syrians were tortured, including with a device which former detainees said was used to stretch or bend victims' arms and legs. The group also found documents indicating Raqqa residents were detained for legal actions like demonstrating or helping the injured.

Rights groups and opposition activists have long claimed that civilians have been detained arbitrarily, tortured, and sometimes have disappeared since the uprising against Assad's regime began. HRW's findings appear to be one of the largest finds of physical evidence bolstering those claims to date.

"The documents, prison cells, interrogation rooms, and torture devices we saw in the government's security facilities are consistent with the torture former detainees have described to us," said Nadim Houry, deputy Middle East director for HRW.

HRW has been documenting abuses on both sides of Syria's civil war during the 26 months of conflict.

The group says abuses by the Assad regime remain far more deadly, systematic and widespread, including attacks on civilians with indiscriminate battlefield weapons such as widely banned cluster bombs. But the rights group also says rebel abuses have increased in frequency and scale in recent months.

In Raqqa, the group's researchers inspected the State Security and Military Intelligence branches and three other detention centers formerly managed by Criminal Security, Political Security, and Air Force Intelligence. Government forces abandoned all these institutions, which are now controlled by the rebels, the group said.

Four former detainees said that officers and guards tortured them, HRW said.

One of them, identified in the report as Ahmed, a 24-year-old student from Raqqa, told HRW that he and his brother had been detained April 7, 2012 in the city's Military Intelligence branch on charges of participating in peaceful demonstrations. The group said it gave only first names of former detainees in the report for fear their testimonies could subject them or their families to further government harassment.

Ahmed said he and his brother had been beaten and tortured with electricity shocks for several hours a day throughout five days of detention. He told HRW that intelligence officers and prison guards wanted him to give up the names of other protesters.

"The torture started in turns between my brother and me," Ahmed said. "They started torturing him with electricity for three, four hours, and then they threw him in a solitary cell. They wanted me to tell them who used to go out to demonstrate with me . and they would make me hear my brother's screams."

The interrogators also threatened to detain his mother. Ahmed told HRW that the possibility of his mother being harmed made him confess to anything.

"Whatever it is you want, I am with you," he said he had told the interrogators.  "I will fingerprint a white piece of paper, and you write what you want."

Ahmed was ultimately tried in a military court in the northern city of Aleppo. He was released from a civilian prison in Raqqa June 8, 2012, following a court decision to sentence him to time served, the report said.

He joined the rebels after his release and has been with a Raqqa-based opposition group known as the Islamic Front for Unity and Liberation, HRW said.

In one method of torture the HRW report details, the victim is tied to a flat board, sometimes in the shape of a cross. In some cases guards stretched or pulled their limbs or folded the board in half so that their face touched their legs, causing pain.

The group also interviewed five people formerly held by Military Intelligence in Raqqa. They said security services questioned them about lawful activities, such as participating in anti-Assad demonstrations, providing relief assistance to displaced families, defending detainees, and providing emergency assistance to injured demonstrators.

Syria's conflict started as a peaceful uprising in March 2011. It became an armed conflict when opposition supporters took up arms to fight a brutal government crackdown on dissent.

At least 70,000 people have been killed and millions forced out of their homes.

Over the past year Syria gradually descended into lawlessness, with a spike of kidnappings in largely rebel-controlled northern Syria as well as the government-held capital. Residents blame criminal groups that have ties to both the regime and the opposition for the abductions of wealthy residents traveling to Syria from neighboring Turkey and Lebanon.

On Friday, the Syrian Observatory for Human Rights expressed "grave concern" for the two bishops, who were abducted last month and have not been heard of since.

Gunmen pulled Bishop Boulos Yazigi of the Greek Orthodox Church and Bishop John Ibrahim of the Assyrian Orthodox Church from their car and killed their driver on April 22 while they were traveling outside the northern city of Aleppo. It was not clear who abducted the priests. No group has publically claimed it is holding the two clerics.

According to the Britain-based activist group the, the two were picked up at a checkpoint in Kfar Dael by Arabic-speaking foreign fighters believed to be from Chechnya

In a statement, the Observatory called on both sides in the civil war to secure their release.